Convert Numeric Fields to Birth Date

I have 3 multi-choice fields (Month, Day, Year) which contain numbers only and I have converted those fields to Numeric fields. I need to put the three fields together to get the birth date (MM-DD-YYYY). We are doing it this way because users in other countries need to populate the info and we have to make sure they don't enter a date as DD-MM-YYYY. By having the 3 different fields, that allows us to control the output.

I have tried using both the original field values and the numeric field values but can't get the right output. What is the best way to convert multiple choice fields to a date field? Thank you!